With the advent of the use of microwave ovens in addition to conventional ovens, has come the need for packages which are multi-ovenable and which also possess many characteristics as well. The metallic packages used for "TV dinners" in years past are not satisfactory as multi-ovenable packages, since the metallic packages are opaque to microwave radiation and result in uneven distribution of heat to the food and may even result in damage to the microwave oven. Thus, metallic trays have been replaced by polymeric trays in possible multi-oven applications. Packages or trays are also known which are made of a material transparent to microwave energy. See for example, U.S. Pat. No. 4,081,646.
In addition, U.S. Pat. No. 4,351,997 discloses the use of a plastic material for the package to accomplish even heat distribution to the food within a single compartment or tray in a microwave oven.
Multilayer polymeric containers are quite common.
Containers of paperboard or plastics such as (1) amorphous nylon, (2) polypropylene, (3) polyethylene terephthalate (PET), (4) crystalline PET (CPET), and (5) polycarbonate are known, but each plastic has inherent disadvantages as an oven-usable packaging container.
(1) Amorphous nylon, for example, has low gaseous and water permeabilities, which is advantageous, but a container made of amorphous nylon requires a minimum of a three (3) layer structure since the heat distortion temperature (HDT) is moderate. One or more layers of a higher heat distortion material would have to be bonded with adhesive layers, to stiffen the structure. A scrap regrind layer may be included. Also, the processing steps required to process amorphous nylon into a container are twofold: (a) sheet coextrusion followed by (b) forming near melting, in the case of trays. Deeper containers are made by extrusion, blow molding or injection blow molding. The resulting container may be subjected to hot filling (the process where the container is filled with a hot liquid and then sealed while the liquid is hot), however, the container made of solely amorphous nylon is not multi-ovenable, nor is it dimensionally heat stable upon retorting.
(2) Polypropylene has extremely high O.sub.2 and CO.sub.2 gaseous permeabilities, (although its water permeability is very low), making it unsuitable, alone, as a shelf stable, oxygen barrier, food package. Therefore, polypropylene containing food packages generally have a minimum of four layers. Additional layers include an oxygen barrier layer, an adhesive layer, and a regrind layer to accommodate scrap generated in the forming processes which is reused in the structure. Most commerically used containers (trays, bowls, cups, tubs) start from coextruded, multilayer sheet which is subsequently thermally formed into a shape by use of a continuous web forming line having multiple tool cavities.
Polypropylene based containers are not ovenable: heat distortion resistance is such that significant loss of strength and stiffness have already occured at about 250.degree. F. Polypropylene based packages are, however, microwaveable. Numerous commercial products exist in polypropylene containers for microwave rewarming. However, the consumer is warned on all such packages not to rewarm contents in a convection oven. Retort sterilization is practiced for food products in polypropylene based containers. However, certain additional processing steps and/or modifications to the basic polypropylene materials and/or container structure must be made in order to allow such containers to survive retort sterilization in the 250.degree. F. to 270.degree. F. range. Examples include the following:
(1) forming of the container at a temperature above the melt temperature of the polypropylene in order to preclude residual stress (thermal memory) within the polypropylene. If this is not practiced, then residual stresses will be relieved during the retort heating, and result in unacceptable dimensional change (distortion of the container); PA1 (2) use of fillers, such as talc, to increase both stiffness at elevated temperatures and to provide added melt strength to the sheet structure in order to allow melt conveyance without appreciable web distortion prior to melt forming; PA1 (3) use of thick walls (40-50 mils) and/or bellows structures to resist and/or accomodate heat and pressure distorting forces acting on the container during retort; PA1 (4) control over pressure during retorting such that the pressure differential, inside the container as compared with the pressure within the retort (external pressure), is minimized during heat up, hold and cool down of the retort.
Our invention, due to a minimum of 28.degree. C. greater heat distortion temperature, and a crystalline melt point of about a minimum of 50.degree. C. greater for the polyketone polymers precludes the need of any such measures for the melt forming, or solid phase pressure forming of polyketone based retortable containers. Thus, melt forming, the use of fillers, thick walls and complicated retorting equipment (ramped temperature and pressure control) are not required, and form the basis for major advantages in fabrication and performance to both the container manufacturer and food processor.
(3) Amorphous, non-crystalline PET (polyethylene terephthalate) and PETG (glycol-modified polyethylene terephthalate) have moderate gaseous permeability and competitive water permeability, but very low HDT's owing to their lack of crystallinity. This low HDT precludes their use in retort or hot fill processes, and neither material is ovenable.
Unsaturated, low molecular weight polyesters can be compounded as pastes with curing agents, then molded and chemically crosslinked to produce thermoset trays with a high HDT. However, a two-step process is required wherein the paste is (1) press molded and then (2) held for a prolonged heat cure interval in a hot mold, resulting in a very slow, multistep process. Contrasted with thermoplastic processing accomplished in a matter of seconds, the thermosetting process takes minutes. Moreover, the thermoset scrap cannot be reused in this process, adding further substantial cost to the molded part and to the consumer. Finally, this approach does not lend itself to inclusion of other layers of material, thus barrier properties are rather poor for these structures.
(4) Crystalline PET (CPET) has low gaseous and water permeabilities, and may be formed into a container with only a monolayer of CPET required. However, the processing steps required are several. In order that the resulting container is multi-ovenable or hot fillable, a third post-forming step is crucial in order to control the degree of crystallization of the polymer to achieve an acceptable HDT and adequate impact resistance. Moreover, the maximum crystallinity allowed for CPET is about 40%. A crystallinity of greater than about 40% results in a brittle container which breaks easily. CPET is dimensionally heat stable after retort if crystallization extent is above 30%. However, the retort process temperatures will cause additional crystallization to occur in CPET rendering the container brittle.
(5) Polycarbonate has high gaseous permeability, making it unsuitable, alone, as a shelf stable package. A polycarbonate container requires a minimum of a four (4) layer structure because of the high permeability and moderate HDT of polycarbonate. A two step forming process is required and the resulting container is multi-ovenable.
There is therefore a need for a method of fabrication of a thermoplastic polymer container apparatus, from which a food or beverage container can be made which is multi-ovenable, hot fillable, retortable, and rigid but not brittle.